The present invention generally relates to single-walled carbon nanotubes (SWNTs) and, in particular, relates to a method for preferential growth of semiconducting vertically-aligned single-walled carbon nanotubes (VA-SWNTs) for use in field effect transistor devices.
Carbon nanotubes can be conceptually viewed as a graphene sheet rolled up into a nanoscale tube form to produce single-walled carbon nanotubes (SWNTs). There may be additional graphene tubes around the core of a SWNT to form multiwalled carbon nanotubes (MWNTs). Depending on their diameter and helicity of the arrangement of carbon atoms in the nanotube walls, SWNTs can exhibit semiconducting or metallic behavior. Semiconducting SWNTs have been demonstrated to be promising building blocks for constructing various electronic devices for a large variety of applications, ranging from chemical/bio-sensors to molecular electronics. For many of these applications, field effect transistors (FETs) made from SWNTs often play an important role.
One of the setbacks which has prevented widespread application of SWNTs in semiconductor electronics is the coexistence of both metallic and semiconducting carbon nanotubes in the as-synthesized samples. Due to the presence of metallic tubes, FET characteristics (e.g. the on/off ratio and integration uniformity) can become poor and uncontrollable. For example, the on/off ratio is typically less than 10 for FETs based on nonseparated carbon nanotube network films. This ratio is generally too small for any practical application. Therefore, it is important to separate metallic nanotubes from semiconducting SWNTs for constructing nanotube-based semiconductor devices having high performance.
A few separation approaches have been employed, including electrophoresis, tube-type-specific physicochemical modification, and selective elimination of metallic SWNTs by electrical breakdown, laser irradiation or gas-phase plasma etching. However, the problem associated with the coexisting metallic nanotubes has not yet been completely solved as the post-synthesis separation processes can often be tedious and may cause possible contamination or degradation of nanotubes. Therefore, it would be desirable to provide preferential synthesis of semiconducting SWNTs directly on a suitable substrate for direct use in FETs.
It would also be preferable to grow aligned/micropatterned SWNTs for electronic applications as their structure-property can be readily assessed, and they can be effectively incorporated into devices. The vertically-aligned structure can not only provide a well-defined large surface area for the device performance enhancement but can also allow controlled functionalization of nanotubes along the tube length for incorporating multifunctionalities. Although the growth of aligned/micropatterned MWNTs is known, the synthesis of aligned SWNT arrays has not been successful.
Accordingly, there is still a need in the art for a method of providing selective growth of semiconducting VA-SWNTs, and to semiconducting VA-SWNTs which can be directly used as the electronically active materials in FET devices without requiring any purification or separation processes.